It gets complicated when putting the game on steam. And also with social media advertisement. These things don't come for free. Steam charges 90 € for putting your game on Greenlight. Social media websites charge you too, that goes without saying.
And no, we will never charge anything for OC to get these funds. No hassle with taxes is wanted, no need to spend money that has been donated over the amount of what is needed.

So after we spent the money what then? Steam is not the answer. You still need to make your game getting seen on Steam. The Free To Play section on Steam isn't that big. Furthermore, as you can see from the description, people expect these games to have monetisation systems.  Or these games are crappy Early Access titles. There's not much appeal at all to that category. And (at least to my knowledge) there is no section "completely free games, never pay anything".
Apart from just being there on Steam, OC would not be visible. No sale would ever feature OC as we can't give any discount.

Indie games often promote themselves through Indie Bundles. That's no option to us either. Why should someone be charged through a bundle for a game that's completely free?

I agree partially with Clonkonaut, Steam is not the ideal platform for this, since they are commercially oriented. They want to advertise games that sell and generate money for them. And since our game is free and will remain free, we will never be in some promotion section of Steam and they will only accept us because we generate traffic and possible new users for them. On the other hand I agree with you that OpenClonk on Steam will generate some traffic and even 1% of their X million users is more than we have currently!

I have once contact the humble bundle initiative and asked whether they also provide open-source bundles to promote the real Indie games, and they told me they are not planning to do so... Again commercialism dictates their game selection strategy. However, we should try again there since the humble bundle is donation based, which in principle does not affect the freeness of our game.

Regarding Facebook/Twitter/Reddit: these are platforms OpenClonk definitely should be on, but organizing and maintaining this takes a lot of time. My hope always was that some community members would pick up such an initiative and promote the game on these platforms. For developers if you have to dedicate time between a regular job and creating content and promotion, the last one is the least prioritized for a good reason... :(